1. Field of the Invention
This invention relates to electrical heaters and more particularly to electrical heaters for melting of heat reactivatable adhesives.
2. Description of the Prior Art
Electrical heaters having a relatively thick plate such as 0.1 inch or greater and constructed of a metal having relatively high heat capacity and high mass are known. One such heating element is constructed by Safeway Products, Inc., Middletown, Conn., in which a resistance type heating element is etched or printed on a silicone rubber layer which is bound to one surface of the plate. A relatively slow acting thermistor having a thickness of 0.06 inches or greater is then provided over the heating element and spaced therefrom by a rubber silicone layer.
In applications where a heat reactivatable adhesive is to be melted, it may be desirable to provide a heating element which has both a high heat output combined with the features of rapid heating and rapid cooling. With the above described prior art heating element, rapid heating and cooling of the electrical heater was hindered by the relatively thick metal plates having high heat capacity and high mass used in the heater. Furthermore, in prior art electrical heaters, although the importance of fast acting temperature sensing devices is known, only relatively slow acting temperature sensing devices are utilized in combination with the known electrical heaters. Since rapid heating requires a relatively high density of power from the heating elements, it is important that the temperatures obtained be sensed very rapidly in order to shut off the heating element prior to self-destruction. Consequently, the prior art heating elements with their slow reacting heat sensors and high heat capacities are not adapted for rapid heating and cooling.
In the book binding machine described in U.S. Pat. No. 3,973,787 issued Aug. 10, 1976 to Staats et al, a heating plate is described on which the backbone of a booklet cover is positioned such that a layer of adhesive covering an inside portion of the backbone will be melted. The heating element is pivotably mounted to permit rocking of the booklet to allow the book leaves to be slashed into the melted adhesive. With such a system, the use of prior art heaters required a long warm-up time coupled with a long cooling time. Consequently, a one-shot melting operation was time-inefficient and for cooling it was desirable to remove the book from the heating element.
In the laminating machine described in U.S. Pat. No. 3,711,355 issued on Jan. 16, 1973 to Staats et al, heaters are utilized which radiate heat to the surface of a pouch comprised of heat reactivatable adhesives. Again, with such a machine, use of the prior art heaters requires a long warm-up time and thus rendered one-shot operation inefficient.